Image-forming MR methods which utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, do not require ionizing radiation and are usually not invasive.
According to the MR method in general, the body of the patient to be examined is arranged in a strong, uniform magnetic field B0 whose direction at the same time defines an axis (normally the z-axis) of the co-ordinate system on which the measurement is based. The magnetic field B0 produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field) of defined frequency (so-called Larmor frequency, or MR frequency). From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field B0 extends perpendicular to the z-axis, so that the magnetization performs a precessional motion about the z-axis. The precessional motion describes a surface of a cone whose angle of aperture is referred to as flip angle. The magnitude of the flip angle is dependent on the strength and the duration of the applied electromagnetic pulse. In the case of a so-called 90° pulse, the spins are deflected from the z axis to the transverse plane (flip angle 90°).
After termination of the RF pulse, the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z direction is built up again with a first time constant T1 (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T2 (spin-spin or transverse relaxation time). The variation of the magnetization can be detected by means of receiving RF coils which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis. The decay of the transverse magnetization is accompanied, after application of, for example, a 90° pulse, by a transition of the nuclear spins (induced by local magnetic field inhomogeneities) from an ordered state with the same phase to a state in which all phase angles are uniformly distributed (dephasing). The dephasing can be compensated by means of a refocusing pulse (for example a 180° pulse). This produces an echo signal (spin echo) in the receiving coils.
In order to realize spatial resolution in the body, constant magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field B0, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving coils then contains components of different frequencies which can be associated with different locations in the body. The signal data obtained via the receiving coils correspond to the spatial frequency domain and are called k-space data. The k-space data usually include multiple lines acquired with different phase encoding. Each line is digitized by collecting a number of samples. A set of k-space data is converted to an MR image by means of Fourier transformation.
In MR imaging, it is often desired to obtain information about the relative contribution of water and fat to the overall signal, either to suppress the contribution of one of them or to separately or jointly analyze the contribution of all of them. These contributions can be calculated if information from two or more corresponding echoes, acquired at different echo times, is combined. This may be considered as chemical shift encoding, in which an additional dimension, the chemical shift dimension, is defined and encoded by acquiring a couple of images at slightly different echo times. For water-fat separation, these types of experiments are often referred to as Dixon-type of measurements. By means of Dixon imaging or Dixon water/fat imaging, a water-fat separation can be achieved by calculating contributions of water and fat from two or more corresponding echoes, acquired at different echo times. In general such a separation is possible because there is a known precessional frequency difference of hydrogen in fat and water. In its simplest form, water and fat images are generated by either addition or subtraction of the ‘in-phase’ and ‘out-of-phase’ datasets.
Several Dixon-type MR imaging methods have been proposed in recent years. Apart from different strategies for the water-fat separation, the known techniques are mainly characterized by the specific number of echoes (or points) they acquire and by the constraints that they impose on the used echo times. Conventional so-called two- and three-point methods require in-phase and opposed-phase echo times at which the water and fat signals are parallel and antiparallel in the complex plane, respectively. Three-point methods have gradually been generalized to allow flexible echo times. Thus, they do not restrict the angle or phase between the water and fat signals at the echo times to certain values anymore. In this way, they provide more freedom in imaging sequence design and enable in particular a trade-off between signal-to-noise ratio (SNR) gains from the acquisition and SNR losses in the separation. Sampling only two instead of three echoes is desirable to reduce scan time. However, constraints on the echo times may actually render dual-echo acquisitions slower than triple-echo acquisitions. Eggers et al. (Magnetic Resonance in Medicine, 65, 96-107, 2011) have proposed a dual-echo flexible Dixon-type MR imaging method which enables the elimination of such constraints. Using such Dixon-type MR imaging methods with more flexible echo times, in-phase and opposed-phase images are no longer necessarily acquired, but optionally synthesized from water and fat images.
However, the in-phase and out-of-phase images acquired (or synthesized) in the afore-described Dixon-type MR imaging methods suffer from a poor SNR as compared to the water and fat images obtained with these methods. This is due to an averaging effect commonly quantified by the effective number of signal averages (NSA) from which the water and fat images resulting from the Dixon water-fat separation usually benefit, but not acquired in-phase and out-of-phase images, and not or only to a lesser extent synthesized in-phase and out-of-phase images. Moreover, the in-phase images acquired (or synthesized) in the afore-described Dixon-type MR imaging methods suffer from a poor SNR as compared to in-phase images acquired by means of separate, tailored, non-Dixon-type MR imaging methods. A comparable SNR would often only be achievable in prohibitively long scan times.
From the foregoing it is readily appreciated that there is a need for an improved technique for Dixon-type MR imaging. It is consequently an object of the invention to provide a method that enables Dixon water-fat separation with high SNR in particular in the acquired (or synthesized) in-phase images.